Electronic and Structural Changes in Ni_0.5TiOPO₄ Li-Ion Battery Cells Upon First Lithiation and Delithiation, Studied by High-Energy X-ray Spectroscopies

Abstract

Carbon-coated Ni0.5TiOPO4, synthesized with a sol–gel method and used as a negative electrode material in a Li-ion battery, was examined with a range of surface and bulk sensitive high-energy spectroscopic techniques to reveal the mechanism for lithium insertion and electrode/electrolyte reactions as a function of battery cycling. Characterizing the electrode/electrolyte interface during the first lithiation with HAXPES showed a partially irreversible reduction of the Ni atoms while EXAFS results of the bulk material revealed formation of Ni clusters. The large initial irreversible capacity loss is attributed to formation of the Ni clusters and SEI (solid electrolyte interphase). The first lithiation process is found to be more complex than previously suggested, also incorporating partial reduction of Ti4+ to Ti2+. Moreover, XANES results also show the PO4 units in the redox reactions being partially participating in the irreversible reactions. Correlating the HAXPES results to the plateaus in the galvanostatic lithiation/delithiation voltage profile indicates a combined cooperative contribution from Ni and Ti in both plateaus; however, the first plateau is dominated by the reduction of Ti while the second is dominated by reduction of Ni.